1. Field of the Invention
The present invention relates to a rigid thermal insulation and/or buoyancy material for an undersea pipes and their accessories, such as valves and flow rate or pressure regulators, and in particular for undersea pipes conveying fluids that are hot or cold, preferably an undersea pipe for use at great depths, or indeed for bottom-to-surface connections between undersea wellheads and a storage and processing ship anchored on the surface.
In most industrial fields it is desired to have high performance insulation systems in order to maintain the fluids conveyed in pipework at constant temperature so that such transfers between pieces of equipment can be carried out over distances that are long, e.g. distances that may be as much as several hundreds of meters or even a few kilometers. Such distances are commonplace in industries such as oil refineries, liquefied natural gas installations (operating at −165° C.), and undersea oil fields that may extend over several tens of kilometers. Such oil fields are being developed in ever-increasing depths of water, which may be as much as 2000 meters (m) to 3000 m, or even more.
The present invention relates in particular to insulated undersea pipes installed on oil fields at very great depths, or indeed it also relates to bottom-to-surface connection pipes suspended between the sea bottom and a surface ship anchored on said oil field, and also to all types of accessory such as valves, flow rate or pressure regulators, etc.
2. Description of the Prior Art
Crude oil generally leaves a well head at a temperature lying in the range 45° C. to 75° C., or even more, and said well heads are often horizontally several kilometers away from the surface support that receives and processes the crude oil, whereas the water of the sea is at a temperature lying in the range about 3° C. to 5° C. Furthermore, the water depth may reach or exceed 2000 m to 3000 m so attempts are made to keep the crude oil until it reaches the surface at a temperature that is higher than 30° C. to 35° C. in order to avoid forming plugs of paraffin or gas hydrates, which would block production. This therefore requires continuous high performance thermal insulation of the pipe of the bottom-to-surface connection conveying the crude oil.
Numerous types of insulating pipe have therefore been developed, and in particular so-called pipe-in-pipe (PiP) type pipes comprising an inner pipe that conveys the fluid and an outer pipe coaxial around the inner pipe, also referred to as an “outer jacket”, that comes into contact with the surrounding medium, i.e. water. The annular space between the two pipes may be filled with an insulating material, or it may be evacuated (i.e. contain no gas).
Such systems have been developed in order to achieve a high level of thermal performance and specific versions have been developed to operate more appropriately in great depth, i.e. to be capable of withstanding pressure at the sea bottom. The pressure of water is substantially 0.1 megapascals (MPa) (i.e. about 1 bar) for a depth of 10 m, so the pressure that the pipe must be capable of withstanding is thus about 10 MPa, i.e. about 100 bars, at a depth of 1000 m, and about 30 MPa, i.e. about 300 bars, at a depth of 3000 m.
Means are known for insulating external pipes that withstand high hydrostatic pressures and that are therefore suitable for use at great depths of immersion, which means are constituted by practically incompressible solid polymer material coatings based on polyurethane, polyethylene, polypropylene, etc., and that may be present, where appropriate, in the form of a solid tubular sleeve. However such materials present thermal conductivity and thermal insulation properties that are fairly poor, and not sufficient for avoiding the above-mentioned drawbacks associated with the formation of plugs in the event of production stopping in an undersea pipe conveying hydrocarbons.
Rigid insulating materials are also known that present advantageous buoyancy, being constituted by synthetic materials containing hollow microspheres (having a diameter of less than 0.1 millimeters (mm)) or hollow macrospheres (having a diameter lying in the range 1 mm to 10 mm) containing gas and capable of withstanding external pressure, that are embedded in binders such as an epoxy resin or a polyurethane resin, and known to the person skilled in the art as “syntactic” foam. Those syntactic foam insulating materials are used mainly for insulating undersea pipes at great depth, i.e. pipes of the riser or multiriser tower type, e.g. as described in WO 00/29276, WO 2006/136960, WO 2009/138609, or indeed WO 2010/097528. Those foams are extremely expensive to fabricate when they are for use at depths greater than 1000 m, i.e. when they need to withstand pressures of about 100 bars, i.e. 10 MPa, since the necessary microspheres must be sorted and tested in order to be capable of withstanding such pressures. In addition, the fabrication process is very difficult when it is desired to fabricate thick elements, since the curing of the chemicals used is a highly exothermic reaction. The main problem is to slow down the physicochemical reaction while simultaneously extracting the heat that is given off, so as to prevent the reaction from running away, which would run the risk of baking or even burning the bulk of the material, thereby generally leading to a material that is unsuitable for its intended use.
Furthermore, the main fabrication faults that are generally encountered are the result of poor control over the curing process, leading to internal deterioration of the polymer matrix, said defects not always being observable prior to installing and starting to use the undersea pipe. It is then found after a few months of operation at high temperature, and in particular for transferring oil at a temperature lying in the range 20° C. to 90° C. with a very great external pressure (10 MPa per depth of 1000 m of water), that cracks occur in the matrix of the polymer and that the microspheres are damaged, thereby leading not only to significant losses of insulation and losses of buoyancy, but above to the creation of cold points, where cold points are particularly troublesome in the event of production being stopped, since the crude oil then freezes very quickly at such cold points, thereby forming highly localized plugs of paraffin and gas hydrates that it is practically impossible to reabsorb in simple manner.
Those high performance rigid syntactic foam insulating materials are used for insulating the running length of pipes, whether the pipes are resting on the sea bottom or the pipes are bottom-to-surface connection pipes. In contrast, those rigid insulating materials are not easy to use for singular junction elements, sometimes known as “spool pieces”, or “connection pieces”, or indeed “bent junction pipes”, since these pipe elements are generally complicated in shape, presenting a plurality of bends or curves, as described in WO 2010/063922, and they need to be fabricated after the undersea pipes have been laid and the bottom-to-surface connections have been installed.
Furthermore, insulating materials are known that are of greater thermal insulation capacity, i.e. of lower thermal conductivity, which materials are associated with phase change properties. Such insulating phase change materials (PCMB) are used in particular in WO 00/40886 and WO 2004/003424, however such insulating PCMB that are capable of adopting a liquid state need to be confined in an absorbent material, as described in WO 00/40886, or they need to be confined in pouches, as described in WO 2004/003424.
Phase change materials act as means for accumulating heat. They give back the accumulated energy on solidifying by crystallizing, or they absorb such energy on melting, with this process being reversible. These materials therefore make it possible to increase the duration of a stop in production without running the risk of the pipes clogging as a result of premature cooling of their content. Nevertheless, those phase change materials present the drawback of their viscous liquid state encouraging heat losses by convection. Another drawback of said insulating phase change materials is that they necessarily give rise to a change in the volume of the material during a change of phase, and that has consequences on the confinement jacket, which must be capable of accommodating such changes in volume.
Those confined thermally insulating coatings are themselves coated in a semirigid continuous tubular outer jacket. However in the prior art, the described embodiments are restricted to fabricating straight pipes and they are not easily adapted to fabricating pipes with bends as described above. Such embodiments are not easily adapted to making thermal insulation on bent junction pipes because of the structure of the outer jackets, since as described they are not suitable for being deformed so as to remain concentric relative to the inner pipe and they do not make it possible to obtain a substantially constant thickness for the insulating material, in particular in the bend regions.
Other insulating materials that are in the form of a gel have been described, in particular in patents FR 2 809 115, FR 2 820 426, and FR 2 820 752, in the name of the Applicant, and in WO 02/34809. More particularly, such insulating gels are constituted by a complex comprising a first compound presenting high grade thermal insulating properties and acting as a plasticizer, which is mixed with a second compound that provides a structuring effect, in particular by curing, such as a polyurethane compound, with the mixture ending up, after the second compound has cured, as an insulating gel constituted by a matrix of said second compound confining said insulating first compound, the insulating gel as finally obtained greatly reducing convention phenomena, in particular in the event of the first compound being a phase change compound.
Said first compound may itself be a phase change compound such as paraffin, other compounds in the alkane family, such as waxes, bitumens, tars, fatty alcohols, glycols, and still more particularly any compound having a melting temperature lying between the temperature t2 of the hot effluents flowing in the inner pipe and the temperature t3 of the medium surrounding the pipe in operation, i.e. in general a melting temperature lying in the range 20° C. to 80° C.
However said first compound may be an insulating compound that does not change phase, such as kerosene, in an intimate homogeneous mixture with a polyurethane polymer, such that together they are in the form of a gel, as described in WO 02/34809.
In prior embodiments, as a result of their extremely flexible elastomer structure and of their relatively fragile mechanical strength, such gels are fully confined by a flexible or semirigid protective jacket, in particular between an inner pipe made of steel and an outer pipe made of thermoplastic material, both in rectilinear pipe portions and in bent pipe portions, and in particular for the bent junction pipes described above.
In order to do this, preconstituted tubular jackets are prepared that are threaded onto a coaxial inner pipe, and the gel is injected into the annular space after the ends of said annular space between said tubular jackets and inner pipes have been shut off. Other methods of making PiP type coaxial bent junction pipes are described in patent WO 2010/063922.
Such insulating gels thus present the advantage of improved thermal insulating properties, while being easier to work than solid insulating materials, in particular with respect to bent junction pipes or indeed to thermal insulation sleeves as described in WO 2010/049627.
The mechanical strength of such gels is nevertheless not sufficient for them on their own to be capable of withstanding the mechanical stresses on the pipes while they are being handled during manufacture, transport, and installation in site, and indeed throughout their lifetime.
Another drawback of such insulating gels is that said first compound, such as kerosene, tends to be exuded from the cured polymer matrix over its lifetime.